Sandbox
{{#if: {{#request:cat}} |{{#dpl:category={{#request:cat}}}} |{{#form:}} {{#input:type=select|name=cat|{{#dpl:namespace=Category|format=,\n*%TITLE%,,}}}} {{#input:type=submit|value=List members}} {{#formend:}}
{{#if: {{#request:cat}} |{{#dpl:category={{#request:cat}}}} |{{#form:}} {{#input:type=select|name=cat|{{#dpl:namespace=Category|format=,\n*%TITLE%,,}}}} {{#input:type=submit|value=List members}} {{#formend:}}